1. Field of the Invention
The preset invention relates to an image forming apparatus that transfers onto a recording sheet plural color toner images formed by developing units, and relates to a control method and a storage medium therefor.
2. Description of the Related Art
An image forming apparatus such as a copier or a printer includes a main controller for controlling the entire apparatus and a printer controller for controlling a printing unit. In synchronism with a sub-scanning sync signal and a main scanning sync signal which are output from the printer controller, the main controller outputs an image signal (video signal) to the printing unit, to thereby control image printing timings in a sub-scanning direction and in a main scanning direction. In the case of color printing, output timings of the sub-scanning and main scanning sync signals are controlled for adjustment of image writing start positions and for prevention of occurrences of color misalignment.
There has been proposed in, e.g., Japanese Laid-open Patent Publication No. 2006-069063, a method for controlling image writing start positions for respective colors in an image forming apparatus having photosensitive drums and laser units. In this control method, a positional deviation of each beam in the sub-scanning direction is received from the printer controller, and according to the received deviation, a start timing of image output from the main controller is controlled on a per beam basis, so as to reduce the positional deviations of respective beams in the sub-scanning direction.
There is another method for controlling an image writing start position in the sub-scanning direction. In this method, a positional deviation in the sub-scanning direction is counted by counting the main scanning sync signal after the top of a page is detected based on the sub-scanning sync signal, and image output for the next line is started after completion of counting the positional deviation.
With these conventional control methods, however, if a timing in which the main controller starts counting the positional deviation is different between respective beams, a line registration error is caused, even if control is made according to acquired positional deviations in the sub-scanning direction for respective colors. A printed image in which a line registration error is caused is shown in FIG. 19. As illustrated, cyan (C), magenta (M), yellow (Y), and black (K) color plane images are printed on an A4 sheet, and printing timings in the sub-scanning direction are deviated by one line.